In typography, rivers (or rivers of white) are gaps in typesetting which appear to run through a paragraph of text due to a coincidental alignment of spaces. Rivers can occur regardless of the spacing settings, but are most noticeable with wide inter-word spaces caused by full text justification or monospaced fonts. Rivers are less noticeable with proportional fonts, due to narrow spacing. Another cause of rivers is the close repetition of a long word or similar words at regular intervals, such as "maximization" with "minimization" or "optimization".
Ok.
That’s one more 40 year old question I had, that I can now cross off my list.
Had no idea this had a name. Very happy to find out that it does!